Alloying two semiconductors of different bandgaps generally results in a semiconductor of bandgap different from that of either original constituent. Varying the relative compositions of the two constituents will lead to the corresponding change in bandgap. This has long been one of the standard methods of achieving a bandgap (and thus the operating wavelength of an optical device) that is not provided by naturally-occurring semiconductors. Unfortunately, this method of achieving wavelength variability is severely limited by existing methods of growing planar epitaxial heterostructures of semiconductor thin films on a crystalline substrate. Such methods invariably require a close match of lattice constants of the substrate and the materials to be grown (or a means of relieving the strain due to lattice mismatch). The very limited lattice constant mismatch required for growing high quality wafers has been the main obstacle of making semiconductor-based optoelectronic devices (such as lasers, detectors, multicolor detectors and solar cells) with controllable and widely variable (or tunable) operating wavelengths. With the advent of nanowire based technology, such restrictions are removed or are very much relaxed, depending on the method of growth. For epitaxial growth of nanowires, the relaxed requirement of lattice matching has led to the growth of materials with a mismatch as large as 8%. InP, GaAs, and other III-V nanowires have been epitaxially grown on Si (see, Martensson et al., Nano Lett. 4, 1987-1990 (2004)) and InAs and InP nanowires have been grown into nanowire heterostructures (see, Bjork et al., Nano Lett. 2, 87-89 (2002)) despite large lattice mismatches. In addition, nanowires can also be grown using an amorphous substrate as simply a mechanical support, allowing alloy nanowire growth with a much larger range of composition variation than is possible with planar growth technologies.
Multicolor lasing and dynamic color-tuning in a wide spectrum range are critically important in many areas of technology and daily life, such as general lighting, display, multicolor detection, and multi-band communication. Multicolor laser sources have an extremely wide range of applications including color display, general lighting, biological detection, holographic imaging and 3-D projection. Color display itself is important in many aspects of technology and daily life. One important advantage of multicolor lasers for color display is the more widely available color range, or color gamut, compared to the currently existing display technologies based on incoherent sources, such as cathode ray tube (CRT) and organic light emitting diode (OLED). The large spectral linewidth of incoherent light sources degrades the color purity and leads to a small color gamut. As coherent light sources, multicolor lasers render high purity monochromic colors and thus can extend the color range significantly if proper wavelengths are chosen. For lighting and illumination applications, the combination of four separate lasers with specifically selected wavelengths can achieve the large chromaticity range and similar color rendering ability as state-of-the-art LEDs or phosphors, even though lasers contain only a series of narrow emission lines. For many of the above applications that require high power output, multicolor lasers offer great advantages due to the much higher wall-plug efficiency than incoherent LED sources, thus leading to greater energy efficiency. While the importance of multicolor lasers and dynamical color control has been well-recognized for a long time, the realization of such sources has been challenging due to several technology barriers.
Multicolor lasers that are necessary for all these critical applications mentioned above require color ranges with widely separated wavelengths, or even across the entire visible spectrum. Such multicolor lasers are fundamentally different from multimode lasers. A multimode laser is conventionally made of a single semiconductor and relies on cavity structure to generate multiple lasing wavelengths corresponding to various cavity modes. Since these multiple modes are all supported by the same gain material, their separation is limited within the gain bandwidth of a semiconductor, typically in the range of 1-30 nm. Such a wavelength range is smaller than usually desired for display or lighting applications. Thus, multicolor lasers require integration or monolithic growth of multiple gain materials, or semiconductor alloys of different alloy compositions. This requirement poses a formidable challenge for traditional planar epitaxial technology due to the large degree of lattice mismatch typically involved. In addition to material challenges, cavity design is also a crucial issue when multiple gain materials are involved in a single integrated structure. This is because the light emitted by the wide-gap materials will be absorbed by the narrow-gap materials. Thus lasing is typically achieved only in the longest wavelength of the structures.
Although various semiconductor alloy nanowires of different compositions have been achieved under separate growth conditions, it is both challenging and important to achieve a full-range composition variation within a single substrate in a single run of growth. To achieve fully tunable lasing within the entire composition range on a single substrate would be even more appealing and challenging.
Therefore there exists a need in the art to provide semiconductor compositions and structural assemblies which enable continuously tunable bandgap on a single substrate. Further, methods for preparing the same which enable straightforward preparation of such semiconductor structures in a single step are needed in the art.